Method for plasma welding

ABSTRACT

The invention relates to a plasma welding process by means of a free microwave-induced plasma jet, which is generated by means of the following process steps:  
     generation of microwaves in a high-frequency microwave source,  
     guidance of the microwaves in a wave guide ( 1 ),  
     introduction of a process gas at a pressure of p≧ 1  bar into a microwave-transparent tube ( 2 ) which comprises a gas inlet opening ( 4 ) and a gas outlet opening ( 3 ), the process gas being introduced through the gas inlet opening ( 4 ) into the microwave-transparent tube ( 2 ) in such a way that it has a tangential flow component,  
     generation of a plasma ( 7 ) in the microwave-transparent tube ( 2 ) by means of electrodeless ignition of the process gas,  
     generation of a plasma jet ( 17 ) by means of the introduction of the plasma ( 7 ) into the working space ( 16 ) through a metallic expansion nozzle ( 5 ) arranged at the gas outlet opening ( 3 ) of the tube ( 2 ).

[0001] The invention relates to a plasma welding process as described inPatent Claim 1.

[0002] In recent years, many efforts have been made precisely in orderto further increase and develop the performance capability ofconventional plasma welding processes, for example tungsten inert gaswelding (TIG) or metal active gas welding (MAG).

[0003] In the case of TIG welding, an electric arc discharges between anon-melting tungsten electrode and the workpiece, by which means theworkpiece is melted. The electric arc has a divergence angle ofapproximately 45°. This means that the distance between the TIG torchand the workpiece significantly influences the power density and thisis, overall, relatively small. Because of the high thermal conductivityof the metals, a substantial proportion of the heat flows into thesurroundings of the weld seam. In the case of a current strength limitedby the life of the electrode and, therefore, also of limited electricarc power, relatively low welding rates result.

[0004] In the case of various plasma welding processes, the plasma jetcan be constricted by means of water-cooled expansion nozzles, by whichmeans a reduction in the (visual) electric arc divergence toapproximately 10° can be effected. In the case of the technically usualdistances between plasma torch and workpiece, therefore, a higher powerdensity and, as a result, a higher welding rate is achieved at identicalelectric arc power. Due to the more stable and, relative to theconventional TIG process, less divergent plasma jet, a smaller influenceof the welding parameters on the shape of the electric arc isadditionally achieved.

[0005] If, in the case of appropriate electrode arrangement, distinctlymore energy is supplied to the electric arc by increasing the currentstrength, the so-called button-hole effect appears. At appropriatethickness, the workpiece is melted eye-shaped and, in the case ofcontinuous advance of the plasma torch, the melted metal flows aroundthe plasma jet and back together behind it.

[0006] A disadvantageous effect of the process described is that thepossible current strength is limited by the life of the electrodes and,therefore, the welding rate is also limited. The result of this is ahigh thermal loading of the component, extensive heat affected zonesand, in addition, a substantial distortion of the workpiece.

[0007] The technical possibilities for increasing the welding ratefurther have been substantially exhausted. In addition to the resultingeconomic consequences, this has the additional effect that it will beimpossible, in future, to achieve results substantially below thecurrent boundaries for energy per unit length, distortion and thedeterioration in properties due to the relatively extensive heataffected zone. This is, in addition, particularly disadvantageous inthat the property potential of modern, high-strength materials, whoseproperties can only be achieved by means of specific heat treatments,cannot by a wide margin be utilized due to the current state ofdevelopment of the conventional welding processes.

[0008] A further disadvantage of the conventional plasma weldingprocesses consists in the limited accessibility and limited possibilityof observing the welding location. This is due to a relatively largenozzle diameter at a small workpiece distance (approximately 5 mm).

[0009] The object of the invention is to provide a process of plasmawelding in which the disadvantages of the prior art are avoided.

[0010] This object is achieved by the process of Patent Claim 1.Advantageous embodiments of the invention are the subject matter of thesub-claims.

[0011] According to the invention, a free microwave-induced plasma jetis used for the plasma welding. This is generated as follows:microwaves, which are guided in a wave guide, are generated in ahigh-frequency microwave source. The process gas is introduced at apressure of p≧1 bar into a microwave-transparent tube, which comprises agas inlet opening and a gas outlet opening, through the gas inletopening of the tube in such a way that it has a tangential flowcomponent. A plasma is generated by means of electrodeless ignition ofthe process gas in the microwave-transparent tube, which plasma isintroduced into the working space through a metallic expansion nozzlearranged at the gas outlet opening of the tube, by which means theplasma jet is generated.

[0012] Particularly advantageous plasma properties are produced by meansof the electrodeless plasma welding process according to the invention.As an example, the specific enthalpy of the plasma and the associatedplasma enthalpy flux density are increased. In association with this,the temperature of the plasma and the plasma jet is increased.Advantages with respect to an increased welding rate and lower weld seamcosts, relative to the welding processes of the prior art, arise fromthis. The plasma welding process according to the invention thereforeprovides an electrodeless welding process that offers substantialeconomic and use advantages with a simultaneously large breadth ofapplication of the welding process.

[0013] In addition, the properties of the plasma jet are improved interms of a reduced diameter and a reduced jet angle divergence. Inaddition, the cylindrically symmetrical plasma jet propagates, in theprocess according to the invention, in a parallel fashion so that theinfluence of the change in distance between torch and workpiece on theshape of the penetration of the plasma jet into the workpiece isreduced. A further advantage is that by this means, the accessibility tothe plasma jet—introduced by a larger possible distance between torchand workpiece—is improved. With the process according to the invention,distances between torch and workpiece of between 30 mm and 100 mm aretherefore possible at a plasma jet diameter on the workpiece of between1 mm and 3 mm. Power densities above 1.5 10⁵ W/cm² can therefore begenerated with the plasma welding process according to the invention.

[0014] The tangential feed of the process gas into themicrowave-transparent tube supports the generation, according to theinvention, of a plasma jet with low jet angle divergence. Because of theradial acceleration caused by the tangential feed of the process gas,which radial acceleration is further reinforced by the cross-sectionalcontraction of the expansion nozzle in the direction of the nozzleoutlet, the non-uniformly accelerated free charge carriers move in thedirection of the expansion nozzle outlet on continually narrowing spiraltracks, by which means the centripetal acceleration of the chargecarriers increases. This motion is also retained by the charge carriersafter emergence from the expansion nozzle into the working space.Because, due to the different ion and electron mobilities, no chargeneutrality is present locally, an axially oriented magnetic field isinduced in the plasma jet, which field leads to a flow constriction ofthe plasma jet after emergence from the nozzle (z pinch). Themagneto-hydrodynamic effect (MHD effect) is involved in this process.

[0015] A further advantage of the method according to the invention isthat the plasma jet can be generated by means of low-cost and robusthigh-frequency systems, for example magnetron or klystron. With thesehigh-frequency systems, advantageous microwave sources are accessible inthe necessary power range up to 100 kW and in the frequency rangebetween 0.95 GHz and 35 GHz. In particular, microwaves of frequency 2.46GHz can be used because, in this case, microwave sources are involvedwhich are of low cost and are widespread in industry and domesticapplications.

[0016] In the plasma welding process according to the invention,furthermore, the energy efficiency is increased relative to conventionalplasma welding processes. As an example, it is possible to generatemicrowave-induced plasmas in which the coupling of power from theradiation field of the microwave sources is greater than 90%. Inconsequence, an increased energy efficiency by 1.5 times arises relativeto welding processes with high-performance diodes and by 20 timesrelative to laser welding processes.

[0017] The coupling of the high-frequency energy of the microwave sourceinto the relevant process gases, as necessary for the plasma generation,then depends on the electromagnetic material constants of the relevantprocess gases, in particular on the complex dielectric constant ε:

ε=68 ′−iε″  (1)

[0018] The complex dielectric constant is a non-linear function of thetemperature and a linear function of the frequency. The relationshipbetween imaginary part and real part of the complex dielectric constantis designated as the dielectric loss angle φ and defines an absorptionprobability of the process medium for high-frequency energy:

tan φ=ε″/ε′  (2)

[0019] The volume-specific absorption of high-frequency energy by afundamentally high-frequency absorbing medium (a suitable process gas inthe present case) is given as follows:

P _(abs) =πνε″|E| ²  (3)

[0020] ν is the frequency of the absorbed high-frequency radiation withthe electrical field strength E in the absorbing volume. Provided theabsorption losses of the high-frequency radiation in the absorbingvolume can be mainly defined by means of the (frequency-dependent)electrical conductivity σ in (Ωm)⁻¹, magnetic effects being negligible,the following applies:

ε″=σ/2πν  (4)

[0021] The total loss power density which can be converted in anelectrically absorbing medium in the case of entering high-frequencyradiation is therefore given by:

P _(abs) ={fraction (1/2)}σ|E| ²  (5)

[0022] In the case of plasma generation by high-frequency radiation ingases, it is necessary to differentiate between the ignitionprocedure—low electrical conductivity—and the procedure for maintaininga plasma—electrical conductivity of typical plasma gases higher thanthat of the corresponding non-ionized gases by at least three orders ofvalue. Both in the case of the plasma ignition and during operation ofthe plasma, a high local electrical field strength E is generallyhelpful because of the dependence of the convertible loss power densityon the absolute square of the local electrical field strength E.

[0023] Because of the electrodeless plasma generation, there is nolimitation with respect to the process gases which can be employed inthe process according to the invention. The process according to theinvention therefore solves the problem of the prior art that, in thecase of electrode-induced plasmas, reactions occur between the processgases employed and the electrode materials with, for example, theformation of tungsten oxide or tungsten nitride in the case of tungstenelectrodes or the occurrence of hydrogen embrittlement. By the selectionof appropriate gases or gas mixtures suitable for the process, it istherefore possible to increase the specific enthalpy of the plasma inassociation with an improved heat conduction between plasma andworkpiece. In an advantageous embodiment of the invention, it ispossible for powder to be supplied to the process gas before entry intothe microwave-transparent tube. By this means, it is for examplepossible to employ the process according to the invention as a powderbuild-up welding process. It is, of course, also possible to supply thepowder to the plasma jet after emergence from the expansion nozzle.

[0024] Because of the electrodeless plasma welding, the entry ofunwanted electrode material into the weld material is also prevented. Inaddition, a disturbance-free, unmanned and automated welding process ispossible without continuous replacement of wear parts.

[0025] A further advantage of the plasma welding process according tothe invention is that the heat affected zone on the workpiece due to theplasma jet is substantially reduced, which results in a lower heatinput, reduced workpiece distortion and a reduction in the damage to thematerial. In addition, a low-fault welding with respect to smaller edgenotches and less porosity of the weld seam is made possible by means ofthe plasma welding process according to the invention.

[0026] In an advantageous embodiment of the invention, the process gasis introduced into the microwave-transparent tube through a nozzle insuch a way that the process gas flowing into the tube has a tangentialflow component and has an axial flow component directed in the directionof the gas outlet opening.

[0027] In a further advantageous embodiment of the invention, viewed inthe flow direction of the plasma, the metallic expansion nozzle has aconvergent inlet on the plasma side and a free or divergent outlet onthe plasma jet side. By this means, it is possible to improve theproperties of the plasma jet with respect to the reduction in the jetangle divergence. Furthermore, the jet diameter can be limited by meansof the opening cross section of the expansion nozzle. Because of thehigh plasma temperatures, the metallic expansion nozzle can, in anadvantageous embodiment of the invention, be cooled.

[0028] In order to ensure reliable operation and reliable ignition ofthe plasmas necessary for the process according to the invention, thewave guide present for the guidance of the microwaves is, in anadvantageous embodiment of the invention, restricted in cross section.The wave guide is then preferably restricted at the location at whichthe microwave-transparent tube is guided through the wave guide. In anexpedient embodiment of the invention, the wave guide and the tube arethen directed at right angles to one another. The advantage is anincrease in the electrical field strength at the location of thecross-sectional restriction. By this means, the ignition properties ofthe process gas are improved, on the one hand, and the power density ofthe plasma is increased, on the other.

[0029] In a further advantageous embodiment of the invention, it is alsopossible to employ a spark gap for igniting the plasma.

[0030] The invention is explained in more detail below using thedrawings. In these:

[0031]FIG. 1 shows the temperature-dependent enthalpy of a nitrogenplasma calculated by means of statistical thermodynamics,

[0032]FIG. 2 shows, in sectional representation, an appliance forcarrying out the process according to the invention with wave guide,expansion nozzle, microwave-transparent tube and a supply unit for theprocess gas,

[0033]FIG. 3 shows an exemplary expansion nozzle in sectionalrepresentation,

[0034]FIG. 4 shows, in plan view, a supply unit for the process gas.

[0035] Microwave-induced thermal plasmas are, in particular, generatedby means of the process according to the invention. These plasmas arecharacterized by a local thermodynamic equilibrium (LTE) between thevarious enthalpy contributions from the plasma. The total enthalpy ofthe plasma is then determined, depending on the molecular nature of theprocess gases, by the following contributions:

[0036] enthalpy from the degrees of freedom in translation, rotation andvibration,

[0037] enthalpy from dissociation,

[0038] enthalpy from ionization.

[0039] By means of the statistical thermodynamics, thetemperature-dependent total enthalpy H(T) and the temperature-dependentthermal capacity C_(p)(T), which can be determined from this by a firstderivation with respect to temperature, can be calculated. Therespective molecular degrees of freedom have then to be taken intoaccount in the condition totals for the translation, rotation andvibration. The corresponding condition totals can then be calculated, inthe presence of dissociation and ionization, from the respectiveequilibrium constants (not performed in any more detail).

[0040] The calculated temperature-dependent enthalpy of nitrogen plasma,which was generated by means of the process steps according to theinvention, is represented in FIG. 1. The diagram shows a very steeppositive slope of the enthalpy up to a temperature of 20,000 K(logarithmic representation on the ordinate).

[0041]FIG. 2 shows, in sectional representation, an appliance forcarrying out the process according to the invention. The representationshows a microwave-transparent tube 2, which is guided at right anglesthrough a wave guide 1, which transports the microwaves generated by amicrowave source (not shown). The microwave-transparent tube 2 is guidedthrough an opening 14 located at the top of the wave guide 1 and throughan opening 15 located at the bottom of the wave guide 1.

[0042] The microwave-transparent tube 2 has a gas inlet opening 4 forthe process gas and a gas outlet opening 3 for the plasma 7. In theregion 12, in which the microwave-transparent tube 2 extends through thewave guide 1, the plasma 7 is generated by microwave absorption.

[0043] A gas supply unit 6 is connected to the gas inlet opening 4 onthe microwave-transparent tube 2 by means, for example, of a crimpconnection in order to avoid destruction of the microwave-transparenttube. Nozzles (not shown), through which the process gas is fed into themicrowave-transparent tube 2, are present in this gas supply unit 6. Inthis configuration, the nozzles are arranged in such a way that theentering process gas has a tangential flow component and has an axialflow component directed in the direction of the gas outlet opening 3.The process gas is, in particular, guided on spiral tracks within themicrowave-transparent tube. This causes a strong centripetalacceleration of the gas in the direction of the inner surface of themicrowave-transparent tube 2 and causes the formation of a depressionalong the tube axis. This depression, furthermore, also facilitates theignition of the plasma.

[0044] The plasma can be ignited by a spark gap (not shown), for examplean arc discharge or an ignition spark. In the case of optimum matchingof the wave guide system, i.e. maximum field strength of the microwaveat the location of the tube axis, an autonomous plasma ignition is alsopossible.

[0045] A metallic expansion nozzle 5 is fastened at the gas outletopening 3 of the microwave-transparent tube 2. In this configuration,the expansion nozzle 5 is arranged in such a way that the opening 14 ofthe wave guide 1 is closed. In order to fix the microwave-transparenttube 2, a groove or a web 11 is machined into the lower surface of theexpansion nozzle 5. In this configuration, the web 11 only protrudes afew millimetres into the wave guide space, which prevents a disturbanceto the microwave field within the wave guide 1.

[0046] On its lower surface, i.e. on the surface facing toward theplasma 7, the expansion nozzle 5 has a convergent inlet. Due to thisrestriction, the charge carriers in the plasma 7 are further acceleratedas far as the outlet opening 17. The plasma 7 then enters, as a plasmajet 8, into the working space 16 through the outlet opening 17. In thepresent representation, the outlet of the expansion nozzle 5 isrepresented as a free outlet. A divergent outlet is, however, alsopossible.

[0047] The centripetal acceleration of the charge carriers in the plasma7 is continued in the free plasma jet 8 after emergence through theexpansion nozzle 5. Because of the centripetal acceleration of thecharge carriers in the plasma jet 8, an axial magnetic field is inducedin the plasma jet 8, as described in the descriptive introduction, bywhich means the constriction of the flow is also continued beyond theoutlet opening 17 of the expansion nozzle 5. A plasma jet 8 with a smalljet angle divergence is therefore generated.

[0048]FIG. 3 shows, in sectional representation, an exemplary expansionnozzle. A web 11 for fixing the microwave-transparent tube (not shown)is machined onto the lower surface of the expansion nozzle 5. The web 11has, in particular, a circular configuration and has an inner radiuswhich corresponds to the outer radius of the microwave-transparent tube.

[0049] The inlet region 9 of the expansion nozzle 5 has a convergentconfiguration, which leads to an increase in the flow velocity of thecharge carriers of the plasma as far as the outlet opening 17. Theoutlet region 10 of the expansion nozzle 5 has a divergentconfiguration.

[0050] In the case of appropriate pressure relationships between thepressure in the working space 16 and the pressure on the inside 12 ofthe microwave-transparent tube, in the case of appropriate size of theoutlet opening 17 and in the case of an appropriate configuration of theinlet region 9 and the outlet region 10 of the expansion nozzle 5, it ispossible to maintain a plasma jet (not shown) which expands into theworking space 16 with supersonic velocity.

[0051]FIG. 4 represents, in plan view, a gas supply unit for supplyingthe process gas to the microwave-transparent tube 2. Two nozzles 18,which feed the process gas into the microwave-transparent tube 2 in twoopposite directions, are embodied in the gas supply unit 6. By thismeans, a tangential feed of the process gas is achieved.

1. A plasma welding process by means of a free microwave-induced plasmajet, which is generated by means of the following process steps:generation of microwaves in a high-frequency microwave source, guidanceof the microwaves in a wave guide (1), introduction of a process gas ata pressure of p≧1 bar into a microwave-transparent tube (2) whichcomprises a gas inlet opening (4) and a gas outlet opening (3), theprocess gas being introduced through the gas inlet opening (4) into themicrowave-transparent tube (2) in such a way that it has a tangentialflow component, generation of a plasma (7) in the microwave-transparenttube (2) by means of electrodeless ignition of the process gas,generation of a plasma jet (17) by means of the introduction of theplasma (7) into the working space (16) through a metallic expansionnozzle (5) arranged at the gas outlet opening (3) of the tube (2). 2.The process as claimed in claim 1, characterized in that the process gasis introduced into the tube (2) by means of a nozzle (18) in such a waythat the process gas flowing into the tube (2) has a tangential flowcomponent and has an axial flow component directed in the direction ofthe gas outlet opening (3).
 3. The process as claimed in one of thepreceding claims, characterized in that, viewed in the flow direction ofthe plasma, the metallic expansion nozzle (5) has a convergent inlet (9)on the plasma side and a free or divergent outlet (10) on the plasma jetside.
 4. The process as claimed in claim 3, characterized in that themetallic expansion nozzle (5) is cooled.
 5. The process as claimed inone of the preceding claims, characterized in that microwaves in thefrequency range between 0.95 GHz and 35 GHz are employed for thegeneration of the plasma.
 6. The process as claimed in one of thepreceding claims, characterized in that the wave guide (1) directed atright angles to the microwave-transparent tube (2) is restricted incross-section at the location at which the tube (2) is guided throughthe wave guide (1).
 7. The process as claimed in one of the precedingclaims, characterized in that a tube with dielectric properties, in SiO₂or Al₂O₃ in pure form without dotations, is employed as themicrowave-transparent tube (2).
 8. The process as claimed in one of thepreceding claims, characterized in that a spark gap is employed forigniting the plasma.
 9. The process as claimed in one of the precedingclaims, characterized in that powder is supplied to the process gasbefore entry into the microwave-transparent tube (2).